Flat-panel displays, such as organic light emitting diode (OLED) and inorganic emissive displays, of various sizes are proposed for use in many computing and communication applications. In particular, such displays are proposed for use in both indoor and outdoor applications under a wide variety of ambient lighting conditions. Indoor applications have relatively low ambient illumination and require lower levels of display luminance. In contrast, outdoor applications can have a high ambient luminance and may require higher levels of display luminance together with low display reflectance. Moreover, most emissive displays are proposed for use in conditions that can be high, or low, or even non-existent ambient illumination; for example, illumination from outdoor use during the day to night-time use in a dark room.
Current illumination and display visibility standards cite 75,000 lux as a standard for outdoor illumination on a bright and sunny day. Cloudy bright days have an illumination of 16,000 lux, cloudy dull days have an illuminance of 6,000 lux, and a cloudy very dull day has an illuminance of 1,000 lux. Indoor illumination ranges from 0 to 1000 lux. Viewability standards for display devices set the minimum display contrast ratio standard for reading text on a display at three. Other sorts of displayed information, such as images, require a higher contrast ratio, for example, ten.
Given the wide variety of viewing conditions proposed for emissive displays, it is difficult to design a display having suitable contrast ratio. For example, OLED displays rely on the use of conductive electrodes, typically some form of highly reflective metal, to provide current to an emissive layer of organic material. The reflective metals reflect ambient light towards a display viewer, thereby, making the display difficult to view.
An OLED display device includes both light-emitting areas and non-light emitting areas. In an active-matrix display, the non-light emitting areas are typically composed of circuitry such as thin-film transistors, capacitors, drivers, and signal lines. In a passive-matrix display, the non-light-emitting areas are typically composed of signal lines.
One way of improving contrast in emissive display devices is to use a circular polarizer over the display. The circular polarizer includes a polarizer and a quarter-wave plate. The polarizer polarizes ambient light falling on the display, and the quarter wave plate circularly polarizes the linearly polarized light by 45 degrees. Any polarized light that is subsequently reflected back through the quarter wave plate is further rotated by 45 degrees so that its direction of polarization is orthogonal to the polarizer, and hence is substantially completely absorbed by the polarizer. Circular polarizers absorb approximately 50%-60% of the light that passes through the polarizer once. Up to 99% of the ambient light that is specularly reflected back through the circular polarizer may be absorbed. Hence, about 50%-60% of the light emitted by the emissive display device through the circular polarizer is lost, while up to 99% of the ambient light that falls on the surface of the display device is absorbed. Suitable circular polarizers are commercially available, for example, from 3M and are described in the patent literature. See, for example, WO/0210845, entitled “High Durability Circular Polarizer for Use with Emissive Displays”, by Trapani et al., published Feb. 7, 2002, which describes a high durability circular polarizer including an unprotected K-type polarizer and a quarter-wavelength retarder that is designed for use with an emissive display module such as an organic light emitting diode or a plasma display device. However, in darker, indoor surroundings it may be advantageous to have improved luminance rather than reduced display reflectance.
A second means of improving contrast in an emissive display device is to place an absorptive layer such as a light absorbing material or a destructive interference layer within a cavity at the back of the device, for example, on the substrate or an electrode. See for example U.S. Pat. No. 6,411,019, entitled “Organic Electroluminescent Device” issued Jun. 25, 2002 to Hofstra et al. The absorptive layer absorbs the ambient light in addition to any light emitted from the emissive layer of organic materials. However, this approach has the difficulty that most of the light emitted from the emissive layer toward the absorptive layer is lost, thereby, severely reducing the luminance of the display.
A third means of improving contrast in an emissive display device is to provide a matrix of light absorbing material called a black matrix between the light emitting elements and around the edges of the display device. See for example, US Patent Application 2002/0050958, entitled “Contrast Enhancement for an Electronic Display Device by Using a Black Matrix and Lens Array on Outer Surface of Display”, by Matthies, et al., published May 2, 2002. This black matrix approach is capable of significantly reducing the reflectance of the display, but still allows substantial ambient light to be reflected from the display by reflection from the reflective anodes of the light emitting elements. Neutral density filters are also known to improve the contrast of a display device.
The use of louver films comprising alternating light-transmissive and light-absorbing portions as privacy-securing films for display devices has been suggested, e.g., as described in US Patent 2004/0191548, entitled “Sheets for Plasma Display Panels”, by Takemoto, published 30 Sep. 2004, and in WO 2005/092544, entitled “Privacy Securing Film”, by Hwang, published 6 Oct. 2005. Such louver films are typically employed on the top surface of a display device, and while effective at reducing viewing angles, typically have not been suggested for use in displays to improve light output and ambient contrast.
Applicants have determined through experimentation that the image quality of the display device as perceived by a viewer depends not only on the ambient contrast, but also on the luminance of the display; hence there is a need for an emissive display that simultaneously improves both ambient contrast and luminance.